Hg 124 (Extract critical, "above the fold" CSS and place it inline)

This is still work in progress, I'm opening this PR in case someone wants an insight on what's happening.
I prefer we split this out into a separate function so that it is unit testable. Also, since we only have one stylesheet to be lazily loaded, it'd be significantly better selector performance to give that link an id and select it and skip the filter. It's likely that in the future, if we asynchronously load other style sheets with modules, it won't be done in the head like this, but at the time of module load (e.g asynchronously loading web components)